In general, polycarbonate resins include a bisphenol as a monomer component and are widely utilized as a so-called engineering plastic in optical fields of electric⋅electronic parts, automotive parts, optical recording media, lenses, etc., and so on, based on superiority in transparency, heat resistance, mechanical strength, and so on.
The conventional polycarbonate resins have used mainly bisphenol A for the monomer; however, in recent years, polycarbonate resins including isosorbide (hereinafter sometimes abbreviated as “ISB”) as the monomer component are developed. The polycarbonate resins using ISB are excellent in various properties, such as heat resistance, optical properties, etc., and utilization for an optical application, such as a retardation film, etc., a glass substitute application, or the like is investigated (see, for example, Patent Literatures 1 and 2). In addition, an interest is also given in the matter that ISB is a dihydroxy compound obtained from biomass resources and is a carbon-neutral material which does not contribute to an increase of emissions of carbon dioxide even when it is incinerated.
However, ISB is a component having high water absorption properties, and there is a case where a polycarbonate resin including ISB as the monomer component causes such a problem that a molded article thereof is deformed under high temperature and high humidity conditions, or other problems. Then, an improvement of moist heat resistance is investigated by a method, such as copolymerization with a monomer capable of imparting heat resistance or low water absorption properties, etc. (see, for example, Patent Literature 3).
In addition, it is known that polycarbonate resins having a structural unit derived from 6,6′-dihydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobiindane (hereinafter sometimes abbreviated as “SBI”) exhibit properties, such as high heat resistance and low birefringence, optical applications for retardation films, lenses, and so on, are investigated based on those properties (see, for example, Patent Literatures 4, 5, and 6).
In addition, in recent years, demands of transparent resins which are used for optical applications inclusive of molded articles represented by front plates of smartphones or automotive navigation systems, optical lenses, optical films, and optical recording media increase. Among these, the diffusion of a thin flat panel display (FPD) represented by liquid crystal displays or organic EL displays is remarkable, and for the purpose of improving a display quality, such as improvement of contrast or coloring, expansion of a viewing angle, prevention of external light reflection, etc., various optical films are developed and utilized.
For example, in organic EL displays, a quarter-wave plate for preventing reflection of external light is used. In retardation films which are used for the quarter-wave plate, in order to suppress coloring and make it possible to reveal a clear black play, wavelength dispersion properties at a broad band such that ideal retardation properties can be obtained at each wavelength in a visible region are demanded. As those corresponding thereto, for example, there are disclosed retardation films composed of a polycarbonate copolymer including a bisphenol structure having a fluorene ring in a side chain thereof and exhibiting reverse wavelength dispersibility such that the retardation becomes smaller as the wavelength becomes shorter (see, for example, Patent Literatures 7 and 8).